low
explosive, and produces a propelling or heaving effect.

The maximum work that an explosive is capable of producing is
proportionate to the amount of heat disengaged during its chemical
transformation. This may be expressed in kilogrammetres by the formula
425Q, where Q is the number of units of heat evolved. The theoretical
efficiency of an explosive cannot, however, be expected in practice for
many reasons.

In the case of blasting rock, for instance:[A]--1. Incomplete combustion
of the explosive. 2. Compression and chemical changes induced in the
surrounding material operated on. 3. Energy expended in the cracking and
heating of the material which is not displaced. 4. The escape of gas
through the blast-hole, and the fissures caused by the explosion. The
proportion of useful work has been estimated to be from 14 to 33 per cent.
of the theoretical maximum potential.

[Footnote A: C.N. Hake, Government Inspector of Explosives, Victoria,
_Jour. Soc. Chem. Ind._, 1889.]

For the purposes of comparison, manufacturers generally rely more upon the
practical than the theoretical efficiency of an explosive. These, however,
stand in the same relation to one another, as the following table of
Messrs Roux and Sarrau will show:--

MECHANICAL EQUIVALENT OF EXPLOSIVES.

Theoretical Work Relative
in Kilos. Value.

Blasting powder (62 per cent. KNO_{3}) 242,335 1.0
Dynamite (75 per cent. nitro-glycerine) 548,250 2.26
Blasting gelatine (92 per cent. nitro-glycerine) 766,813 3.16
Nitro-glycerine 794,563 3.28

Experiments made in lead cylinders give--
Dynamite 1.0
Blasting gelatine 1.4
Nitro-glycerine 1.4

Sir Frederick Abel and Captain W.H. Noble, R.A., have shown that the
maximum pressure exerted by gunpowder is equal to 486 foot-tons per lb. of
powder, or that when 1 kilo, of the powder gases occupy the volume of 1
litre, the pressure is equal to 6,400 atmospheres; and Berthelot has
calculated that every gramme of nitro-glycerine exploded gives 1,320 units
of heat. MM. Roux and Sarrau, of the Depot Centrales des Poudres, Paris,
by means of calorimetric determinat